For most of the history of integrated circuit manufacture, chip sizes were standard, and the package for those chips was typified by a large dual-in-line package (DIP) having long leads for insertion through holes in printed wiring boards.
In recent years, integrated circuit packaging has changed significantly. Lead count has grown, and pitch correspondingly decreased. Leads are now typically designed for surface mount or solder bump mount.
While the size of the chips has generally increased, the dimensions of the overall package have decreased. Among the consequences of this trend is a problem of bowing of the package. Bowing causes distortion of the plane of the leads, and interferes with mounting of the package on a circuit board. Bowing of packages with solder bumps is particularly serious, and frequently renders the parts useless in planar assembly.
The cause of bowing in thin packages is differential thermal contraction between the chip and the plastic encapsulant. The flexural rigidity of the thin package is significantly less than the thicker ones. Hence, disparities in the coefficients of thermal expansion coefficients between the lead frame and the silicon die can cause far greater bowing. In addition, significant differences in thickness of encapsulant above and below the device are more common in thin packages and make matters worse. With thick packages of the prior art, flexural rigidity of the thicker plastic was sufficient to resist the stress.